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Coffee's Bitter Mystery

BOSTON--People who start their mornings by pouring cream and sugar into a steaming mug of coffee are usually trying to mask the beverage's bitterness. But the reason coffee makes us pucker has eluded scientists for decades. Now, researchers have narrowed the search by identifying two chemical compounds responsible for bitter taste in coffees ranging from mild breakfast blends to intense espressos. According to work presented here at the annual meeting of the American Chemical Society, it turns out that the roasting process--not the raw beans--produces these compounds, a finding that could open the door to improved methods for processing coffee beans.

A cup of coffee is a complex brew of more than 30 chemical compounds that contribute to its taste, aroma, and acidity. Since the 1930s, scientists have separated and identified numerous chemicals responsible for many of the sensory components of a cup of joe, but few have investigated those that produce bitterness.

To explore further, food chemist Thomas Hofmann of the Technical University of Munich in Germany and colleagues sequentially filtered brewed coffee. They found that the fractions containing the lowest molecular weight compounds were the most bitter, giving the team a target for further analysis. Using mass spectroscopy, the researchers identified one of the compounds as chlorogenic acid lactone, a breakdown product of chlorogenic acid, which is present in nearly all plants. Next, the team brewed a series of coffees from light to dark roasts and measured the amount of chlorogenic acid lactone in each.

The researchers found that roasting coffee beans sets off a cascade that converts the chlorogenic acids in raw beans first to chlorogenic acid lactones and then, if roasting continues, to breakdown products of the lactones called phenylindanes. Although the lactones produce the mild bitterness characteristic of a light or medium-roast coffee, the second breakdown product is primarily responsible for the harsh, bitter taste characteristic of a dark-roasted bean, Hofmann says.

The identification of the secondary breakdown product adds to a growing body of literature investigating the potential health effects of coffee drinks, says Peter Martin, a professor of psychiatry at Vanderbilt University in Nashville, Tennessee, and director of the university's Institute for Coffee Studies. "As little as we know about the [chlorogenic] acids, we know even less about these other products," he says. "Identifying them expands our horizons of what we need to look at to better understand the healthful effects of coffee and for the industry to understand the permutations of taste."